being a 540 owner & a tech at Benz I would say that problem wise they are pretty much the same, but some parts on the 540 cost more than some on Benzs, also expect to replace some clutches on the 540, they tend to go more often.
the price seems a little low, is there anything wrong with the car?, accidents?

but when it all comes down to it, BMW's are more fun to drive, they handle much better, it's a sportier car